1. Field of the Invention
The present invention relates to a technology for embedding digital watermark information in document data in such a way as not to be easily detected visually, a technology for detecting embedded digital watermark information and a technology for preventing important information from being leaked from a printed paper medium.
2. Description of the Related Art
Recently, the digitalization of information stored in an organization, such as an enterprise and the like has been promoted and storage by paper is anticipated to decrease in the future. However, since it does not necessarily mean no paper document, currently, continuously generated paper and digital data are mixed as an inevitable result. On the other hand, since it is feared that private information, such as customer data handled by enterprises and public institutions and the like, is leaked, the introduction of a stricter management system is required.
As one security technology for coping with such a situation, digital watermark is used. The digital watermark is a technology for embedding information, such as the name of a copyright holder, reproduction history and the like in data, such as an image, document, voice and the like. By embedding such digital watermark information in important data handled within an organization, products provided outside an organization, information leakage and its reproduction can be prevented or the original can be assured.
For example, a technology for inserting watermark information by giving the criteria of change and its priority on the basis of a human visual characteristic against a binary image and changing a pixel with higher priority first is disclosed (for example, see Patent Document 1).
A technology for detecting watermark information by embedding watermark information by detecting a text area from a document image and increasing/decreasing the amount of features of the detected text area, or dividing the detected text area into a plurality of small sections, further dividing the small sections into two or more groups and increasing/decreasing the amount of features against each group and comparing the integrated value of the amount of features obtained from each group is disclosed (for example, see Patent Document 2).
Although as described above, the study of digital watermark for digital data is popular and various inventions are commercialized, it is difficult to extract embedded information from document data, especially a paper medium printed out once. This is because embedded information is easily lost due to position deviation, noise addition, the loss of high frequency components by resolution conversion and the like, caused when printing scanning.
Patent Document 1: Japanese Patent Publication No. 2004-289783
Patent Document 2: Japanese Patent Publication No. 2001-78006